Protective coating



United States Patent 3,053,691 PROTECTIVE COATING Harry J. Hartman and Walter E. Pocock, Baltimore Md.,

assignors to Allied Research Products, Incorporated, 5 Baltimore, Md, a corporation of Maryland No Drawing. Filed Jan. 29, 1958, Ser. No. 711,777 14 Claims. (Cl. 117-75) This invention relates to the treatment of metals to provide the same with a protective corrosion-resistant and abrasion-resistant coating which also has excellent paint bonding characteristics and, more particularly, to an improved coating composition and method for forming the coating.

A simplified flow diagram of the process is as follows: 15

Alloy such as aluminum alloy Cleaned J Metal such as steel Cleaned Electroplated such as zinc plated Riused with Water Treated with chemical conversion coating solution Rinsed with water Treated with acidic solution of polyacrylarnide and water soluble chromium compound Dried at room temperature Dried at 120 to 250 F.

Previously used organic coatings necessitate the use of 55 organic solvents and where water soluble resins are used,

inhibitor or inhibitors, to coat such metals. -It is within the scope of this invention that the polyacrylamide can be added directly to the solution used for generating a chromate conversion coating on the metal surface, codepositing with the chromate coating. This aspect of the invention is limited in application to metals upon which a conversion coating can be formed. The corrosion resistance, water repellency, and paint bonding characteristics are thereby improved.

A further object of this invention is to provide a corrosion protective film on metals and alloys which, if desired, can be removed quickly and easily by immersion in a hot alkaline solution; retaining the clean oxide-free surface available as it was prior to treatment. This is particularly important where a temporary protective film is desired.

The invention further comprehends the use of polyacrylamide plus a corrosion inhibitor or inhibitors in aqueous solution as a simple dip brush or spray operation whereby a coating can be applied to a metal surface or to a surface having a previously formed conversion coating, without any objectionable chemical reaction taking place on the surface.

It is possible by use of this invention to use organic and inorganic coloring agents to add color to the film.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art fro-m this detailed description.

While a polyacrylamide coating on the metal by itself has little water resistance and a chomate coating by itself, likewise, has little water resistance, it has been found that if a mixture of the polyacrylamide and chromate is employed under acid conditions, the coating obtained has outstanding water resistance. The film is insolubilized simply by drying in open air at room temperature, although the insolubilization procedure can be ac celerated by hot drying.

We find that polyacryl-amide itself possesses little or no corrosion protective value over metals, but, surprisingly, when a corrosion inhibitor or inhibitors are combined with the polyacrylamide, there results a greatly improved corrosion protection over the use of the corrosion inhibitors alone, as well as the polyacrylamide, so that the condition is remedied. iln addition, it has been found that by utilizing the polyacrylamide with the corrosion inhibitor, a greatly reduced amount of corrosion inhibitor can be employed to obtain the same amount of protec tion.

In brief, the principal components of the coating which form the subject of the present invention are soluble in acid solutions.

The preferred corrosion inhibitor, or inhibitors, used in this invention are water soluble chromium compounds such as chromic acid, sodium, potassium, ammonium dichromate, and the corresponding chromates, as well as less soluble complex chromium-chromate gels containing both hexavalent and trivalent chromium, made, for example, by adding sodium chromate to chromium nitrate under slightly alkaline conditions, and very slightly soluble chromium compounds such as zinc chromate and strontium chromate. Such compounds are excellent corrosion inhibitors when used in conjunction with the polyacrylamide in accomplishing the scope of the invention.

When used as a supplementary coating, the polyacrylamide and inhibitors previously mentioned can be mixed dry and then dissolved in water at the time of application to the metal or they can be premixed with water. The aqueous mixtures can have a pH of 1 to 11, desirably on the acid side, and preferably pH 1 to 3. To maintain the pH, there can be used any acid or acid salt or alkali or alkali salt that does not reduce the chromate too rapidly for practical application. Typical acids which can be employed are hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, acetic acid, formic acid, chromic acid, etc., and as alkalis there can be utilized sodium, potassium or ammonium hydroxide, or carbonates, such as sodium and potassium carbonates.

The invention also includes the addition of polyacrylamide directly to baths employed for the formation of the chromate conversion coatings. Corrosion resistance, paint bonding characteristics and abrasion resistance are thereby improved. Chromate conversion coatings referred to here are formed by immersion of a metal in a solution comprising hexavalent chromium compounds, and an activator or activators in an acid solution. The metal is dissolved at the interface, the reaction with the metal resulting in a chromate gel which is precipitated on the surface. When polyacrylamide is incorporated in the formulation this is coprecipitated with the chromate gel improving the characteristics, e.g., corrosion resistance, paint bond properties and abrasion resistance, of the film formed.

In what follows, unless otherwise indicated, all parts and percentages are by weight.

The aqueous compositions of the present invention contain 0.05 to 5 parts of polyacrylamide per 100 parts of water. If the polyacrylamide is the film former, as previously described, then it is used in an amount of 0.2 to 5 parts per 100 parts of water. The chromate or other corrosion inhibitor is used in admixture with the polyacrylamide in the amount of 0.01 to parts of inhibitor per 100 parts of water.

If the chromate conversion bath is the primary film former, then only 0.05 to 1 part of polyacrylamide is normally used per 100 parts of water in the aqueous mixture containing 0.01 to 50 parts of chromate as hexavalent chromium.

One of the advantages of the instant invention is that the compositions are relatively low in cost.

Temperatures of 60 to 100 F. are preferably employed in the coating process. Temperatures as high as 200 F. can be employed, however. It is surprising that such high temperatures can be used since it would be expected that there would be quantitative decomposition of the polyacrylamide in the presence of acidified chromates, a strong oxidizing system, at elevated temperatures, e.g., 180 F.

Accelerated drying can be carried out at 120 to 250 F., preferably 140 to 180 F. When acid solutions of polyacrylamide are employed, the drying insolubilizes the film, particularly if the solution is applied over a chromate conversion coating.

The polyacrylamide used in this invention is usually of such a molecular weight range that a 4% aqueous solution has a viscosity varying from 60 to 600 centipoises at 77 F., and preferably 100 to 250 centipoises under these conditions. In the following examples, the polyacrylamide used has a viscosity of 200 centipoises at 77 F. at a 4% concentration.

As dry compositions, there can be used mixtures of polyacrylamide and inhibitor or inhibitors in the following proportions:

Parts Polyacrylamide 100 Inhibitors 0.2 to 100 A, or mixtures of polyacrylamide and chromate conversion composition mixtures in the following proportions:

Parts Polyacrylamide 0.1 to 50 Chromate composition Example 1 A steel test panel electroplated with zinc and at room temperature was immersed in the following aqueous solution having a temperature of 80 F. and immediately withdrawn, the panel being covered with liquid and then dried at 160 F..

Polyacrylamide percent 2 Chromic acid, C1-C do 0.2 pH 2 A clear abrasion-resistant coating was formed. The test panel showed only slight edge corrosion after 68 hours exposure in a standard 20% salt spray. A similar panel, zinc plated only, was severely corroded after 24 hours salt spray exposure.

Example 2 Example 1 was repeated using half the specified concentration. Corrosion in salt spray was slightly less than with Example 1, but still an improvement over zinc plate alone.

Example 3 Example 1 was repeated using double the specified concentrations. Corrosion in salt spray was somewhat greater than with Example 1.

Example 4 A steel panel was zinc plated, rinsed with water, immersed in the following aqueous solution at 80 F. and then dried at 160 F..

Polyacrylarnide percent 2 Sodium bichromate, Na Cr O .2H O -do 0.2 pH 6 A clear abrasion-resistant coating was formed. After 24 hours salt spray, the panel was only slightly corroded as compared to severe corrosion on a similar panel that was zinc plated only.

Example 5 A steel panel was zinc plated, rinsed with water and subjected to the following procedure:

(1) Immersed for 20 seconds, at 80 F., in an aqueous solution containing:

hours salt spray exposure, the panel treated by steps 1-4 was markedly less corroded than a panel that had been treated by steps 1, 2, and 4 only. Steps 1, 2, and 4 are an example of a typical commercial chromate conversion treatment designed to improve the corrosion resistance of zinc surfaces.

Example 6 An aluminum test panel of 3003 alloy was cleaned, immersed in the solution of Example 4, and dried at 160 F. A clear abrasion-resistant coating was formed. After 66 hours salt spray exposure, the panel was markedly less corroded than a similar panel that had been cleaned only.

Example 7 An aluminum test panel of 3003 alloy was subjected to the following procedure:

(1) Cleaned,

(2) Rinsed with water,

(3) Immersed for 3 minutes, at 80 F., in an aqueous solution containing:

Chromic acid, CrO percent 0.05 Potassium ferricyanide, K Fe(CN) do 0.01 Sodium fluosilicate, Na SiF do 0.014 Barium nitrate do 0.02

(4) Rinsed with water, (5) Immersed in the solution of Example 4, and (6) Dried at 160 F.

Example 8 A steel panel was cleaned and immersed in the following aqueous solution at 80 F. and dried at 160 F.:

Percent Polyacrylamide 2 Sodium chromate, Na CrO 1 A clear abrasion-resistant coating was formed. After 1 hour salt spray exposure, the panel was only slightly rusted, as compared to severe rusting of a similar panel that had been cleaned only.

Example 9 Example 8 was repeated, using double the specified concentrations. The resistance to rust in salt spray was somewhat greater than with Example 8.

Example 10 To one liter of an aqueous acidic solution for passivating and improving the corrosion resistance of zinc surfaces and containing 10 grams of Na Cr O .2H O, 10 grams of H BO 2.3 grams of NaCl and 1 ml. of 42 B. HNO was added 2 grams of polyacrylamide.

A hot-dipped galvanized zinc sheet was immersed in this bath 5 seconds at 140 F. and then dried at 70 F. The coating obtained had improved corrosion resistance as determined by humidity tests, compared to a similar coating from which the polyacrylamide was omitted.

Example 11 Example 10 was repeated utilizing 0.25 gram of the polyacrylamide. Corrosion resistance was less than that obtained in Example 10; however, there was a measurable improvement over the coating from which the olyacrylamide was omitted.

Example 12 Example 10 was repeated utilizing 5 grams of polyacrylamide with comparable results. Polyacrylamide can -be added as above indicated to any of the formulations disclosed in Hartman Patent No. 2,799,601. The performance of said formulations is improved by the polyacrylamide addition.

An aluminum test panel of 3003 alloy was subjected to the following procedure:

(1) Cleaned,

(2) Rinsed with water,

(3) Immersed for 3 minutes, at F., in an aqueous solution containing:

Chromic acid, CrO "percent-.. Potassium ferricyanide, K Fe(CN) do Sodium fluosilicate, Na SiF do Barium nitrate do Polyacrylamide do pH (4) Rinsed with water,

(5) Dried at room temperature.

The panel was then painted with a white baked enamel, along with a panel similarly treated, except that the polyacrylamide was eliminated from the solution of step 3. .The adherence of the-paint to the metal surface was markedly improved by the presence of olyacrylamide as compared to no polyacrylamide.

After 500 hours salt spray exposure, there was noticeably less corrosion of the metal and less blistering and flaking of the paint where polyacrylamide was included as compared to no polyacrylamide, indicating the beneficial effect of polyacrylamide on corrosion resistance.

Example 14 Example 13 was repeated except using 0.5% polyacrylamide instead of 0.1%. There was a further improvement in initial paint adherence and in corrosion resistance as indicated by 500 hours salt spray exposure.

Referring to Examples 13 and 14, polyacrylamide can be profitably added to any of the formulations disclosed in Ostrander et a1. Patent No. 2,796,371.

Example 15 Zinc metal sheet was dipped into an aqueous solution containing:

Chromic acid, CrO 7 grams/liter. Sodium sulfate 1 gram/liter. Nitric acid, 42 B 3 ml./liter. pH 0.3.

The sheet was allowed to remain in the solution until a visible conversion coating was formed (about 15 seconds). The sheet was then removed, rinsed in water and immersed in an aqueous solution containing:

Polyacrylamide 2.0%. Hydrochloric acid, 20 B 1.7 ml./1iter.

The sheet was then dried at 160 F. A clear waterresistant coating was formed which was superior in abrasion resistance to the conversion coating itself.

Example 16 Example 1 was repeated but the aqueous polyacrylamide solution was maintained at 160 F. The results obtained were similar to those in Example 1.

We claim:

1. A composition containing 0.05 to 5 parts of polyacrylamide and 0.01 to 10 parts of a hexavalent chromium compound.

2. A composition according to claim 1 wherein the chromium compound is present in an amount of 0.05 to 0.5 part.

3. An aqueous composition containing 0.05 to 5 grams of olyacrylamide and 0.01 to 10 grams of a hexavalent chromium compound per parts of Water.

4. A composition according to claim 3 having a pH of not more than 5.

5. A method of impanting a corrosion-resistant coating to a metal which comprises subjecting the metal to an aqueous solution containing polyacrylamide and a hex-avalent chromium compound.

6. A method according to claim 8 wherein the polyacrylamide is present in an amount of 0.5 to 50 grams per liter of solution and the hexavalent chromium compound is present in an amount of 0.1 to 100 grams per liter of solution.

7. A method according to claim 6 wherein the solution has a pH not greater than 5.

8. A method of imparting increased abrasion resistance to a metal having a coating containing a hexavalent chrmium compound as a corrosion inhibitor comprising subjecting the metal having said initial coating to an aqueous solution containing polyacrylamide and a hexavalent chromium compound.

9. A method according to claim 8 wherein the polyacrylamide is present in an amount of 0.5 to 50 grams per liter of solution and the hexavalent chromium compound is present in an amount of 0.1 to 100 grams per liter of solution.

10. A method of imparting increased abrasion resistance to a metal having a corrosion-resistant chromate conversion initial coating comprising subjecting the metal having said initial coating to an aqueous solution containing polyacrylamide.

11. A method according to claim 10 wherein the polyacrylamide is present in an amount of 0.5 to 50 grams per liter of solution.

12. A method of imparting a corrosion-resistant coat- 8 ing to a metal which comprises subjecting the metal to an aqueous solution containing polyacrylamide and a hexavalent chromium compound and thereafter drying the coated metal.

13. A method according to claim 12 wherein the aqueous solution has a pH not greater than 5.

14. A method of imparting increased abrasion resistance to a metal having a corrosion-resistant chromate conversion initial coating comprising subjecting the metal having said initial coating to an aqueous solution containing polyacrylamide and thereafter drying.

References Cited in the file of this patent UNITED STATES PATENTS 2,335,962 Pollack Dec. 7, 1943 2,411,590 Powell Nov. 26, 1946 2,412,528 Morrell Dec. 10, 1946 2,508,717 Jones May 23, 1950 2,636,257 Ford Apr. 28, 1953 2,709,664 Evans May 31, 1955 2,744,080 Brant May 1, 1956 2,902,390 Bell Sept. 1, 1959 2,921,858 Hall Jan. 19, 1960 2,930,106 Wrotnowski Mar. 29, 1960 OTHER REFERENCES Schildkrecht: Vinyl and Related Polymers, John Wiley and Sons, Inc. (1952), pages 314317. 

8. A METHOD OF IMPARTING INCREASED ABRASION RESISTANCE TO A METAL HAVING A COATING CONTAINING A HEXAVALENT CHROMIUM COMPOUND AS A CORROSION INHIBITOR COMPRISING SUBJECTING THE METAL HAVING SAID INITIAL COATING TO AN AQUEOUS SOLUTION CONTAINING POLYACRYLAMIDE AND A HEXAVALENT CHROMIUM COMPOUND. 